Scope of (string) literals - c

Question

I always try to avoid to return string literals, because I fear they aren't defined outside of the function. But I'm not sure if this is the case. Let's take, for example, this function:
const char *
return_a_string(void)
{
return "blah";
}
Is this correct code? It does work for me, but maybe it only works for my compiler (gcc). So the question is, do (string) literals have a scope or are they present/defined all the time.

Answer 1

This code is fine across all platforms. The string gets compiled into the binary as a static string literal. If you are on windows for example you can even open your .exe with notepad and search for the string itself.
Since it is a static string literal scope does not matter.
String pooling:
One thing to look out for is that in some cases, identical string literals can be "pooled" to save space in the executable file. In this case each string literal that was the same could have the same memory address. You should never assume that it will or will not be the case though.
In most compilers you can set whether or not to use static string pooling for stirng literals.
Maximum size of string literals:
Several compilers have a maximum size for the string literal. For example with VC++ this is approximately 2,048 bytes.
Modifying a string literal gives undefined behavior:
Modifying a string literal should never be done. It has an undefined behavior.
char * sz = "this is a test";
sz[0] = 'T'; //<--- undefined results
Wide string literals:
All of the above applies equally to wide string literals.
Example: L"this is a wide string literal";
The C++ standard states: (section lex.string)
1 A string literal is a sequence
of characters (as defined in
lex.ccon) surrounded by double quotes, optionally beginning with the
letter L, as in "..." or L"...". A string literal that does not begin
with L is an ordinary string literal, also referred to as a narrow
string literal. An ordinary string literal has type "array of n
const
char" and static storage duration (basic.stc), where n is the
size
of the string as defined below, and is initialized with the given
characters. A string literal that begins with L, such as L"asdf",
is
a wide string literal. A wide string literal has type "array of
n
const wchar_t" and has static storage duration, where n is the size
of
the string as defined below, and is initialized with the given charac-
ters.
2 Whether all string literals are distinct (that is, are stored in
nonoverlapping objects) is implementation-defined. The effect
of
attempting to modify a string literal is undefined.

Answer 2

I give you an example so that your confusion becomes somewhat clear
char *f()
{
char a[]="SUMIT";
return a;
}
this won't work.
but
char *f()
{
char *a="SUMIT";
return a;
}
this works.
Reason: "SUMIT" is a literal which has a global scope.
while the array which is just a sequence of characters {'S','U','M','I',"T''\0'}
has a limited scope and it vanishes as soon as the program is returned.

Answer 3

This is valid in C (or C++), as others have explained.
The one thing I can think to watch out for is that if you're using dlls, then the pointer will not remain valid if the dll containing this code is unloaded.
The C (or C++) standard doesn't understand or take account of loading and unloading code at runtime, so anything which does that will face implementation-defined consequences: in this case the consequence is that the string literal, which is supposed to have static storage duration, appears from the POV of the calling code not to persist for the full duration of the program.

Answer 4

Yes, that's fine. They live in a global string table.

Answer 5

No, string literals do not have scope, so your code is guaranteed to work across all platforms and compilers. They are stored in your program's binary image, so you can always access them. However, trying to write to them (by casting away the const) will lead to undefined behavior.

Answer 6

You actually return a pointer to the zero-terminated string stored in the data section of the executable, an area loaded when you load the program. Just avoid to try and change the characters, it might give unpredictable results...

Answer 7

It's really important to make note of the undefined results that Brian mentioned. Since you have declared the function as returning a const char * type, you should be okay, but on many platforms string literals are placed into a read-only segment in the executable (usually the text segment) and modifying them will cause an access violation on most platforms.